Among the methodologies used to synthesize high-porosity aerogel architectures, Freeze-Drying (or Ice-Templating) represents a processing route that avoids the high-temperature and high-pressure regimes required by supercritical fluid drying. The operational principle of freeze-drying relies on the solid-liquid phase transition of a solvent (typically water or tert-butanol) at reduced temperatures, utilizing the resulting solid ice crystals as removable physical templates. As ice crystals nucleate and grow within an aqueous suspension, dispersed nanoparticles, polymer chains, or two-dimensional nanosheets are rejected by the advancing solidification front and concentrated into the interstitial spaces between adjacent ice crystals. Subsequent sublimation under reduced pressure converts the solid ice directly into gas, leaving behind a three-dimensional porous aerogel that mirrors the morphology of the ice template.

The physical foundation of this process lies in controlling ice crystal growth kinetics. The nucleation frequency and kinetic growth rate of ice crystals within an aqueous suspension are governed by the degree of thermodynamic supercooling at the solid-liquid interface, alongside localized thermal and mass transport rates. If freezing proceeds under isotropic thermal conditions, ice crystals nucleate randomly and grow omnidirectionally, producing an isotropic aerogel with unordered pore topologies after sublimation.

By applying a directional thermal gradient—such as placing a cryogenic cold source at the base of the suspension opposed by a thermal boundary above—researchers control the spatial direction of ice crystal growth. Driven by this unidirectional thermal flux, ice crystals nucleate at the cold interface and grow as parallel dendrites or lamellae aligned with the temperature gradient axis. Through ice-front propulsion effects, the suspended nanomaterials are pushed into the parallel interstitial gaps between advancing ice fingers, assembling into aligned two-dimensional wall structures or one-dimensional fibrous arrays.

The physical competition between the solidification front velocity ($v$) and the particle diffusion velocity governs the topological architecture of the resulting aerogel cell walls. According to interfacial physics models, when the ice front velocity $v$ remains below a critical particle rejection velocity threshold $v_c$, the suspended particles are expelled by the advancing ice front, forming smooth honeycomb or lamellar pore walls. Conversely, when $v > v_c$, particles are engulfed by the moving solidification front and embedded inside the ice crystals, producing fractured or disordered pore walls after sublimation.

By tuning freezing velocities, thermal gradient slopes, solvent compositions, and structure-directing additives (such as sugars or alcohols) that modify ice crystal growth habits along specific crystallographic planes (e.g., basal vs. prism faces), materials scientists customize anisotropic aerogel micro-topologies—including parallel honeycomb, helical, or hierarchical cellular structures. Utilizing ice-crystal phase transition kinetics to construct anisotropic topological networks provides a physical route to bypass the mechanical and transport limits of traditional isotropic aerogels.